Doubled diffused metal-oxide-semiconductor field effect transistor (DMOSFET), insulated gate bipolar transistor (IGBT), and Schottky diode are important power devices and use extensively as output rectifiers in switching-mode power supplies and in other high-speed power switching applications. For example, the applications include motor drives, switching of communication device, industry automation and electronic automation. The power devices are usually required carrying large forward current, high reverse-biased blocking voltage, such as above 30 volt, and minimizing the reverse-biased leakage current. There are several reports that trench DMOS, trench IGBT and trench Schottky diode are superior to those of with planar structure.
For power transistors are concerned, apart from the device in the active region for carrying large current, there is still required a termination structure design in the periphery of the active region usually at an end of a die so as to prevent voltage breakdown phenomena from premature. Conventional termination structures include local oxidation of silicon (LOCOS), field plate, guard ring, or the combination thereof. The LOCOS is generally known to have bird beak characteristic. In the bird beak, electric field crowding phenomena is readily to occur, which is due to high impact ionization rate. As a result, leakage current is increased and electrical properties of the active region are deteriorated.
For example, please refer to FIG. 1, a semiconductor substrate with trench MOS structure for Schottky diodes, and a trench termination structure formed therein. The substrate is a heavily doped n+ substrate 10 and an epitaxial layer 20 formed thereon. A plurality of trench MOS 15 formed in the epitaxial layer 20. The trench MOS devices including epitaxial layer 20/gate oxide layer 25/polysilicon layer 30 are formed in the active region 5. The boundary of the active region 5 to the edge of the die is a LOCOS region of about 6000 .ANG. in thick formed by conventional method.
For the purpose of lessening the electric field crowding issue, a p+-doping region 50 beneath LOCOS region is formed through ion implantation. The p+-doping region 50 is as a guard ring for reverse-biased blocking voltage enhancement. The anode (a metal layer) 55 is formed on the active region 5 and extends over p+ doping region 50 of LOCOS region. The object is to make the bending region of the depletion boundary far away from the active region 5. Although guard ring 50 can alleviate the electrical field crowding and relax the bending magnitude occurred near the active region, the adjacent region between p+ region 50 and beneath the bottom of the trench MOS device, as arrow indicated denoted by 60, is not a smooth curve. It will increase the leakage current and decrease the reverse-biased blocking capability. A similar situation occurred for field plate combines with guard ring. Furthermore, aforementioned prior art demanded more photo masks (at least four) to fabricate, and the processes are rather complicated. Still high cost for forming such structure is another inferior.
As forgoing several conventional termination structures can not solve the problems thoroughly. An object of the present invention thus proposes a novel termination structure. The new termination structure made the bending region of the depletion region far away from the active region, and depletion boundary is flatter than forgoing prior art. The manufacturing method provided by the present invention is even simpler than those prior arts. Since the termination structure and trench are formed simultaneously, it requires only three photo masks, low complicated processes and low cost.